1. Field of the Invention
The present invention relates to a process for the preparation of a readily water-redispersible and water-wettable polymer powder by spray-drying of an aqueous dispersion of polymer particles with admixing of a hydrophilic and of a hydrophobic antiblocking agent, wherein the spray-drying of the aqueous polymer particle dispersion is carried out in the presence of a hydrophobic antiblocking agent and the polymer powder obtained is homogeneously mixed with a hydrophilic antiblocking agent in a subsequent step.
2. Description of the Background
In many applications, polymers which can be incorporated into an aqueous medium in a simple manner are required. In many cases, aqueous dispersions of polymer particles (aqueous polymer dispersions) which can often be used directly are suitable for this purpose. A disadvantage of aqueous polymer dispersions, however, is that, with a water content of up to 60% by weight, they require large volumes during storage and, on transfer to the customers, water cheaply available everywhere also has to be transported at high costs in addition to the desired polymer.
This problem is frequently solved by subjecting the aqueous polymer dispersions, which are obtainable, inter alia, by a free radical aqueous emulsion polymerization familiar to a person skilled in the art, to a spray-drying process, likewise familiar to a person skilled in the art, for the preparation of corresponding polymer powders.
With the use of these polymer powders, for example as binders in adhesives, sealing compounds, synthetic resin renders, paper coating slips, surface coating compositions and other coating materials or as additives in mineral binders, the polymer powders must as a rule be redispersed in water. This can be effected either by redispersing the polymer powder in water and using the resulting aqueous polymer dispersion for mixing with the other formulation components or by mixing the polymer powder together with the other formulation components with water. In both cases, it is important that the polymer powder forms the original polymer particles again rapidly and without formation of agglomerates when brought into contact with water. The basis for this is the instant behavior of the polymer powder used in water, which behavior comprises the redispersing behavior and the wetting behavior of the polymer powder.
The redispersing behavior is an important property for the quality of the polymer powder. The better the redispersing behavior of the polymer powder in water, the more closely the properties of the aqueous polymer dispersion after redispersing approach the properties of the aqueous polymer dispersion prior to the spray-drying step. This means that the redispersing behavior of the polymer powder is a measure of the extent to which the original and the redispersed aqueous polymer dispersions correspond in their properties.
If the polymer powder also has good wetting behavior, the aqueous polymer dispersion can form during the redispersing even without the use of an intensive mixing technique, which has advantages in practice.
While the redispersing behavior of a polymer powder is generally substantially influenced by the spraying assistants usually used in the spray-drying process and therefore familiar to a person skilled in the art, the wetting behavior is determined by the surface characteristics of the polymer powder particle. This is frequently determined by the antiblocking agent adhering to the surface of the polymer powder particle.
As antiblocking agents, DE-A-2614261 discloses, for example, finely divided silicas, which are used in amounts of from 4 to 20% by weight, based on the polyvinyl acetate/ethylene copolymers. However, powders produced in this manner have a large fine fraction and therefore tend to form dust. Furthermore, these polymer powders tend to agglomeration on introduction into water, which agglomeration can be eliminated only by an intensive mixing process.
DE-A-2614261 describes, in a similar process, the use of from 2 to 20% by weight of silica as an antiblocking agent for polyvinyl acetate and other copolymers of vinyl acetate. Here too, the abovementioned disadvantages are applicable.
DE-A-1719317 discloses, as antiblocking agents, aluminum silicates, kieselguhr, clays, calcium sulfate, cements or diatomaceous earth, which are mixed with the polymer powder after the spray-drying process, the abovementioned disadvantages being applicable here too.
Silicas rendered hydrophobic by reaction with chlorosilane are disclosed in DE-A 3101413 as advantageous antiblocking agents in the preparation of vinyl ester powders.
EP-A 1000113 describes a process for the preparation of redispersible and wettable polymer powders, in which a hydrophilic and a hydrophobic antiblocking agent are to be used simultaneously for the spray-drying. This process is said not to have the abovementioned disadvantages. A disadvantage of this process, however, is that large amounts of antiblocking agents are added during the spray-drying process, which, owing to the finely divided nature of the antiblocking agents used, frequently leads to deposits or to blockages of the waste air pipes or separation cyclones or waste air filters, with the result that the spray-drying process is susceptible to faults.